The Failing Human Machine - Biomaterials: A Tantalus Experience - page 23

endoprosthesis was an example of a poor mechanical design. This prosthesis was
designed for complex peri- and fractures. The proximal bulky part was developed
for substituting defect bone. Fracture sometimes occurred in the proximal part of
the slender stem just below the bulky cubic part. The section of the stem was in this
case probably not sufficient to withstand the bending moment at that particular side
of the stem [45]. More historic cases of this kind are known but all documentation
has disappeared.
Inclusion : Another reason for fracture could be a material defect, something like
an inclusion or an impurity just at the surface which acts as a notch for the onset of
fracture. An aluminum oxide particle could, for example, have penetrated the sur-
face during sand blasting. Inspection by optical microscopy and scanning electron
microscopy, preferentially equipped with an energy dispersive X-ray fluorescence
spectrometer (SEM-EDX) of the fracture surfaces are the indicated detection tools
here. Analysis of the case of Fig. 2.1 learned us, however, that it was not the case
here. An example of an inclusion that caused a fracture initiation is discussed by
Helsen and Breme in their topic Metals as Biomaterials [35, Chap. 15].
Fracture occurs when a critical stress is exceeded but a notch can lower this
critical stress. A notch can have many origins: careless manufacturing, an inclu-
sion at or just underneath the surface...Rapid loading of notched pieces (a fall of
a patient) can provoke progress of the notch into catastrophic fracture. More about
rapid loading in [50].
Fatigue : Joints of hip or knee and their replacements are typically all cyclically
loaded structures. Consequently, the materials concerned were all subject to fatigue .
The stress-strain curve of Fig. 1.6 ends at a stress by which a crack propagates
catastrophically and results in fast fracture of the test specimen. Cracks, however,
can grow gradually at stresses much lower than the UTS, the stress at the catas-
trophic end of stress-strain curve! In Fig. 2.4 , the explanted prosthesis is shown and
the fractured surface in Fig. 2.5 .
Fig. 2.4 The explanted broken Charnley prosthesis of Fig. 2.1 . The lower part of the stem is heav-
ily corroded. On the fracture surface, the progressing crack ( lower left ) and the ductile fracture
( upper right on the fracture surface) can be clearly distinguished